The present invention relates to an apparatus for determining the point or points at which drill pipe, casing, tubing, or the like is stuck in an earth bore hole. For convenience, the term pipe will be used hereinafter to include casing, tubing, drill pipe, and the like. Although there are many reasons for determining the location at which a pipe is stuck, the principal reasons relate to the removal of the free pipe above the stuck point or the performance of various operations at the stuck point with or without the removal of the pipe. Devices used for determining the stuck point are commonly called freepoint indicators.
The approximate location of the stuck point may be estimated by stretching the free pipe above the stuck point and predicting the location of the stuck point from the physical manifestation of the pipe at the surface. Once the approximate location of the stuck point is known, a freepoint indicating device is lowered into the pipe to locate the actual stuck point. The freepoint indicating device generally consists of a pair of spaced members connected by a sensing means that permits a limited amount of relative movement between the spaced members and produces a signal representing the relative movement. Each of the spaced members includes an anchor assembly that allows the individual member to be connected to the pipe.
When a stuck point or freepoint measurement is to be made, each individual spaced member is connected to the pipe in some manner and a force applied to the upper portion of the pipe. For example, the force may be torque applied to the upper end of the pipe to twist the pipe with the individual spaced members rotating relative to each other or remaining stationary depending upon whether the torque applied to the upper end of the pipe has produced relative rotation of the portions of the pipe engaged by the members. It will be understood that when the torque is applied to the upper end of the pipe, the length of the pipe from the upper end thereof down to the stuck point will twist and that the applied torque will not twist the pipe below the stuck point. Therefore, the operator may lower the device progressively down through the pipe in a step by step order applying a twist to the pipe as each new position of the device in the pipe is established thereby determining for each position of the device whether or not the twist applied to the upper end of the pipe has produced relative rotation of the individual spaced members. When the device passes the stuck point, no relative rotation of the individual spaced members will occur. The operator could, of course, start the test near the lower end of the pipe below the stuck point where no relative rotation of the spaced members of the device occurs and conduct the measurements consecutively upward until the device is brought into a position where the torque applied to the upper end of the drill pipe produces relative rotation to the spaced members.
After the stuck point is located, it may be desirable to remove the free pipe above the stuck point from the well. One method of accomplishing this is by locating the first free joint above the stuck point. A predetermined amount of torque is applied to the pipe and an explosive charge exploded at the free point to break the joint allowing the free pipe to be unscrewed and removed from the well. Under other conditions a device for severing the pipe is positioned at a point above the stuck point. The pipe is severed and the section of pipe thus released withdrawn from the well.
Numerous freepoint indicators have been disclosed in prior art. One such freepoint indicator is disclosed in U.S. Pat. No. 3,934,466 which employs a transducer for measuring the relative longitudinal movement of the upper and lower sections of the device. U.S. Pat. No. 4,105,071 is directed to a freepoint indicator which uses two sensing means, one sensing means to detect angular deformations and the other sensing means to detect longitudinal deformations of a pipe string. Additional freepoint indicators of the prior art are those disclosed in U.S. Pat. Nos. 2,530,309; 2,534,632; 2,851,880; 3,670,566; and 3,762,218.